Structural and spectroscopic characterization of the brownmillerite-type Ca 2 Fe 2−x Ga x O 5 solid solution series

Abstract

Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca 2 Fe 2 O 5 induced by the substitution of Fe 3+ with Ga 3+ . 61 synthetic single-crystal samples of Ca 2 Fe 2−x Ga x O 5 0.00 ≤ x ≤ 1.328 have been investigated by single-crystal X-ray diffraction at 25 °C. We find that pure Ca 2 Fe 2 O 5 and samples up to x ~ 1.0 have space group Pnma, Z = 4, whereas samples with x > 1.0 show I2mb symmetry, Z = 4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm −1 . These Raman modes are obviously linked to changes in the Ca–O bond lengths at the phase transition. 57 Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga 3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca 2 Fe 2−x Ga x O 5 solid solution compounds with 0 ≤ x ≤ 1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga 3+ contents are paramagnetic. Over and above, the substitution of Fe 3+ by Ga 3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20–720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca 2 Fe 2 O 5 end-member.